Most oil and gas wells are drilled using drill pipe. After a certain depth is reached, the operator runs a string of casing into the open borehole and cements the casing in place. The operator may then run drill pipe into the casing to drill below the casing to a greater depth. In a casing-while-drilling technique, all or a portion of the well is drilled using the casing as the drill string. The operator mounts a bottom hole assembly to the lower end of the casing. The bottom hole assembly includes a drill bit and a reamer. The operator may rotate the casing, which in turn rotates the drill bit to drill the well. The operator may also employ a mud motor that rotates the drill bit relative to the casing in response to drilling mud pumped down the casing.
Prior to reaching the total desired casing setting depth, the operator may wish to retrieve the bottom hole assembly, such as to replace a worn drill bit. Also, when reaching the casing setting depth or total depth, the operator may wish to retrieve the bottom hole assembly rather than cement it in place. One technique for retrieving the bottom hole assembly is to lower a wireline through the casing, latch it to the bottom hole assembly, then retrieve the wireline along with the bottom hole assembly. While successful, this technique requires special equipment not always found on a drilling rig, such as a wireline winch and sheaves to guide and lower the wireline into the casing. Another technique involves pumping the bottom hole assembly up the casing by reverse circulating. However, some operators fear that reverse circulation will damage the open hole formation. Another technique involves running drill pipe through the casing, latching the drill pipe to the bottom hole assembly, then retrieving the drill pipe along with the bottom hole assembly. Most drilling rigs will have equipment of this nature available.
Running the drill pipe and tripping it out to retrieve a bottom hole assembly takes a number of hours, depending on the depth of the well. A possibility exists that the casing may become stuck in the well while the drill pipe is tripped in and out. This can particularly be a problem if the operator plans to drill deeper with the string of casing. To avoid the casing sticking, it would be desirable to circulate up the outer annulus between the casing and the bore hole from time to time. Also, reciprocating the casing up and down reduces the chances of it becoming stuck.
Another issue while retrieving and re-running a bottom hole assembly is to provide a safety barrier in the event that a formation begins to flow liquid or gas, referred to as a “kick” while the drill pipe is located inside the casing. It is known that if such a kick occurs while running casing, the kick can normally be controlled by circulating fluid through the casing. However, no arrangement presently exists that allows circulation through the casing while a string of drill pipe is being run in or retrieved inside casing.